Ianh wrote:

Adding to Blackwoifs drilling method, if you have an SDS drill with roto stop , I.e it can be in hammer mode with no rotation , you can grind a sharp edge on an SDS chisel bit and use drill in roto stop mode to chisel the nut after drilling it.
It makes things easier than swinging a big hammer in tight spaces and applies many hard blows per minute.

This works a treat on exhaust nuts so should work well for you.

I tend to initially chisel directly towards the bolt, then as it splits chisel at an angle to undo the nut.

NB, this is in essence what you would do in a workshop with an air chisel, but something you can do at home if you , a friend , or neighbour have an SDS drill with roto stop.

Ps, when drilling use a centre punch to start the drill in the centre of the nut , especially if drilling parallel to the bolt. Use a 3 mm drill bit to start, smaller and the drill bit will likely break, bigger and it will take forever. Use plenty of oil or grease on drill bit. Once drilled with 3mm move to 4mm, the 4.5 etc , just making sure you don’t catch the bolt threads. Then chisel.

Many thanks above most closely matches what I did. started with a 3mm drill. generally started on the flat face of nut but also resorted to some parallet drilling and chisel.
when you get the drill speed just right and the wind blowing in the right direction its very satisfying getting along spiral of metal swarf coming off the drill

I've got two more to get off and them I'm onto the rebuild.

oh. I also used a 4" grinder extensively on some nuts - in this instance I wasn't worried about damaging thread, which is just aswell